1. Field of the Invention
This invention relates to an optical system for compensating for the shake of a photographic image to obtain a stable image and, more particularly, zoom lenses for stabilizing the image by decentering a lens group constituting part of this optical system wherein the decentering aberrations, which are produced when the lens of this part is decentered, are suppressed, thus providing zoom lenses which enable a good optical performance to be obtained.
2. Description of the Related Art
When taking shots from on a running car, flying aircraft or other moving bodies, vibrations propagate to the photographic system, causing the photographic image to shake.
In the past, image stabilizing optical systems having the function of preventing image shaking have been proposed in, for example, Japanese Laid-Open Patent Application No. Sho 50-80147, Japanese Patent Publication No. Sho 56-21133 and Japanese Laid-Open Patent Application No. Sho 61-223819.
In the Japanese Laid-Open Patent Application No. Sho 50-80147, for a zoom lens having two afocal variable magnification systems, letting the angular magnification of the first variable magnification system be denoted by M.sub.1 and the angular magnification of the second variable magnification system by M.sub.2, variation of the image magnification is performed by each variable magnification system while having the relationship: M.sub.1 =1-(1/M.sub.2). Along this zoom lens, the second variable magnification system is spatially fixed to compensate for the shake of the image. Thus, stabilization of the image is achieved.
In the Japanese Patent Publication No. Sho 56-21133, responsive to the output signal from the detecting means for detecting the vibrating state of the optical device, an optical member which is part of the optical device is made to move in such a direction as to cancel the vibratory displacement of the image due to the vibrations, so that stabilization of the image is achieved.
In the Japanese Laid-Open Patent Application No. Sho 61-223819, a photographic system is formed to include a refraction type variable angle prism arranged on the most object side and the vertex angle of the refraction type variable angle prism is made to vary in correspondence with the vibration of the photographic system to deflect the image. In such a way, stabilization of the image is achieved.
In other documents such as Japanese Patent Publication No. Sho 56-34847 and Japanese Patent Publication No. Sho 57-7414, an optical member constituting part of the photographic system is arranged to be spatially fixed against vibrations. By utilizing the prism effect this optical member generates for the vibrations, the photographic image is made to deflect so that the stable image is obtained on the image plane.
Also, an image stabilization method is known where an acceleration sensor is utilized to detect the vibrations of the photographic system. Responsive to the signal obtained in real time, part of the photographic system, or a lens group, is made to oscillate in a direction perpendicular to the optical axis.
In a mechanism for oscillating the lens group constituting part of the photographic system to remove the shake of the photographic image to obtain the stable image, the photographic system is generally required to be of such a form that the lens group movable for compensating for the shake of the image is small in size and light in weight, the relationship between the amount of compensation for the shake of the image and the amount of movement of the movable lens group is simplified, and the time necessary for computing the transformation is shortened.
Also, if decentering of the movable lens group allows large decentering coma, decentering astigmatism and decentering curvature of field, etc. to be produced, the compensation for the shake of the image will, because of the decentering aberrations, result in blurring of the image. For example, the decentering distortion, when increased largely, causes a shifted amount of the paraxial portion of the image and a shifted amount of the marginal portion of the image to become different from each other. For this reason, with the paraxial portion of the image being aimed, when the movable lens group is decentered to compensate for the shake of the image, it is in the marginal portion that a similar phenomenon to the shake of the image begins to take place. This causes an extreme lowering of the optical characteristics.
For this reason, for an image stabilization photographic system, particularly a variable magnification optical system, it is desired that when the movable lens group is moved in the direction perpendicular to the optical axis to effect decentering, the amount of decentering aberrations produced will be small, the lowering of the optical performance will be slight, and the structure of the operating mechanism will be simple.
However, a photographic system which can satisfy all the above-described various conditions is, in general terms, very difficult to obtain. Particularly when provision for decentering is made in that part of the photographic system which is a lens group having a refractive power, the optical performance is largely lowered, thus creating the drawback that a good quality image is sacrificed.
Related art of the invention are disclosed in U.S. patent application Ser. Nos. 261,231 filed on Oct. 24, 1988 and 346,512 filed on May 2, 1989, etc.